Age-related hearing loss, presbycusis, is one of the most common ailments of the elderly. Presbycusis is often treated with hearing aids which serve to amplify some or all of those sound frequencies lost to peripheral hearing loss. However, hearing aids have mixed results in the elderly population because presbycusis, in addition to the peripheral hearing loss, involves age and deprivation-induced plastic changes to the auditory brain. As a result, reintroducing sound with a hearing aid does not necessarily (or immediately) restore normal hearing to the elderly listener. Indeed, clinicians have become aware of a delay in the effectiveness of hearing aids after fitting patients, a process known as acclimatization. This delay is frustrating for many hearing aid users and may result in rejection of the device in a subset of elderly patients. How this acclimatization process works is poorly understood, but it is clear that central plastic changes resulting from wearing hearing aids "definitely do occur" (Syka, 2002). The proposed study will address what happens to neurons in the aged brain when sound frequencies are reintroduced to a presbycusic mammal. Aged CBA mice (n=15) with presbycusis will be exposed to three months of low-level (70dB SPL), frequency-specific acoustic stimulation (12 hours/day) while control mice (n=15) matched for age, auditory behavior and hearing thresholds, will be reared in normal vivarium conditions. The effects of the treatment will then be assessed by measuring auditory behavior (Specific Aim 1), primary auditory cortex electrophysiology (Specific Aim 2) and GABA neurochemistry in primary auditory cortex (Specific Aim 3). The overall goal of this research is to gain a better understanding of the plastic changes that occur in the presbycusic brain following reintroduction of sound. Such information might be useful in understanding the plastic potential of the aged central auditory system and might eventually aid in the development of more effective therapeutic treatments for elderly patients with presbycusis.